1. Field of the Invention
This invention relates to the production of printed circuit assemblies, and more particularly, to the vision inspection of mounted printed circuit components for defect identification and resolution.
2. Description of the Prior Art
A printed circuit assembly may have a large number and variety of components mounted on one or both sides of a printed circuit board. After component installation, but before final soldering, it is desirable to inspect each component-bearing side of the board for correct component assembly. This entails a check for wrong components, reversed components, broken components, mis-wired components or wire jumpers, and other defects.
Component-side inspection of circuit boards is often performed manually, either with or without the aid of optical magnification. Manual inspection is based on the inspector""s familiarity with the product or by comparing the printed circuit assembly against a known reference such as a document, photograph, or an actual known good assembly. Such comparisons tend to be tedious and error prone because the inspector must repeatedly alternate attention between the reference and the board being tested (xe2x80x9cTest Boardxe2x80x9d). As a result, manual component-side inspection of circuit boards is usually performed on a sample basis or is performed as a first board check before running many new boards.
As an alternative to manual inspection, automatic vision systems using roving cameras or line scan imagers have been utilized in some printed circuit assembly inspection applications. In an automatic system, many similar printed circuit assemblies are presented to the system such that it can learn the acceptance and rejection criteria. Optical character and shape recognition may also be used in conjunction with board data such as CAD generated information. Such systems can be very complex and prohibitively expensive. They are most effective in low product mix/high product volume environments.
A third approach to printed circuit assembly inspection, representing a compromise between manual and automatic inspection systems, is Blink Overlay Comparison. According to this method, alternating images of a known good board (xe2x80x9cGolden Boardxe2x80x9d) and a Test Board are repeatedly presented to a human operator. The two images appear in alternating fashion at the same visual position in the field of view of the observer. If the two boards are exactly alike and their images are presented to an observer in alternating fashion at some rate (such as five times per second), then the observer will not notice any movement of the images. On the other hand, if there are any differences between the two boards, then only the parts of the images with the differences will flash at the animation rate. These differences are generally due to the assembly defects mentioned above. By displaying the two images in the same visual position, the movement of the observer""s attention and focus is eliminated. This increases the speed and accuracy of the inspection.
Several approaches to Blink Overlay Comparison have been used in the past. One such implementation is based on a pure optical comparison. In this method, two optical imaging systems are used in conjunction with a shutter system. The Golden Board is fixtured in a frame at a precision location under the lens of one of the imaging systems. The Test Board is similarly fixtured under the lens of the other imaging system. The two fixtures that hold the boards are coupled mechanically so that they can be moved simultaneously in X-Y directions under their respective lenses. The images from the two lenses are alternately displayed through a common optical eyepiece as in a microscope. Alternation between the two images is achieved with a motorized shutter system. Because the two boards are fixtured using a common reference, such as their respective tooling holes, their images are presented in the same relative position in the eye piece as they appear during alternate intervals.
In another implementation of Blink Overlay Comparison, the optical imaging systems described above are replaced with standard broadcast-grade color video cameras. The optical viewing system is replaced with a color monitor. The fixturing for the Golden Board and the Test Board is similar to that used in the optical system. The images of the two boards are electronically switched and alternatingly displayed on the video monitor.
Both of the foregoing Blink Overlay Comparison systems suffer from several basic problems. First, these systems require that the Golden Board be physically present. This means that an inventory of Golden Boards needs to be maintained. As an example, if a facility produces 100 different types of boards at an average cost of $500.00, then a $50,000 inventory of Golden boards is tied up at all times. Second, both the Golden Board and the Test Board must be placed very accurately in their respective fixtures so that the two images line up properly. Third, due to the limited resolution of the optical and video systems, only a very small portion of the Golden Board and the Test Board can be viewed at one time. This requires many movements of the two boards under the lenses.
One prior art system overcomes certain of these problems. It includes a downward looking video camera that is mounted on an X-Y positionable mounting assembly and a precision board-holding fixture. First, a Golden Board is placed on the precision board holder using the Golden Board""s tooling holes. This provides a repeatable position for the Golden Board and the subsequently imaged Test Board. The video camera has a limited resolution, for example, 500xc3x97500 pixels. In order to read the value of components on a board for comparison purposes, the camera has to be zoomed in so close that on a typical 16xe2x80x2xc3x9716xe2x80x2 board, only a very small portion (such as 4xe2x80x2xc3x974xe2x80x2) of the board is captured at one time. In that case, the camera moves and grabs images at 16 different locations on the Golden Board. In this way a library of images for each Golden Board is created and stored.
When running the production of any one board type, the Golden Board file for that board is loaded. In order to compare a newly manufactured Test Board, it is placed in the precision board holder in the same manner as the Golden Board and the imaging sequence is started. The camera moves through the 16 slices, according to the example above, stops at each position, and animates between the Golden Board and Test Board images on a color monitor for comparison. The system then moves the camera to the next slice upon a signal from the operator.
By digitizing and storing the Golden Board images, this prior art system eliminates the need for an inventory of actual Golden Boards. However, there are several remaining problems with this design. First, the board holder is a precision fixture that locates the Golden Board and the Test Board by engaging fixture pins into the tooling holes in each board. Therefore, various fixtures need to be built for different types of boards. Second, the fixturing of a board requires that it be taken off-line and placed in the fixture. This slows production. Moreover, fixturing can be difficult unless the board is already soldered and there are no loose components that can be disturbed or dropped. However, the ideal place to inspect for assembly defects is before soldering when the defects can be easily corrected. Third, due to the low resolution of the camera, an elaborate zooming and x-y camera positioning system is required. This adds complexity and can slow down system operation.
Accordingly, there is a need in the art for a vision comparison inspection system for printed circuit assemblies that overcomes the foregoing disadvantages of the prior art. What is required is a system that allows rapid and efficient inspection of circuit board assemblies in a production line environment without the need for time-consuming fixture set ups and complicated camera zooming and x-y positioning manipulations.
A vision comparison inspection system is provided in accordance with the present invention that solves the foregoing problems and provide an advance in the printed circuit assembly inspection art. The inspection system is easily incorporated into a printed circuit assembly production line having a plurality of processing locations and a conveyor system for transporting circuit assemblies between processing locations in an upstream to downstream work flow direction. In a preferred embodiment of the invention, the inspection system includes a printed circuit assembly image capture and inspection conveyor disposed in the production line. The image capture and inspection conveyor is adapted to receive printed circuit assemblies from an upstream portion of the production line and to transport the printed circuit assemblies to a downstream portion of the production line for subsequent processing. An electronic imaging device is fixedly positioned to capture an image of a printed circuit assembly positioned on the image capture and inspection conveyor at an image capture location. A lighting system is mounted for illuminating the printed circuit assembly at the image capture location. A position control system positions the printed circuit assembly at the image capture location. An imaging control system, including a programmed image processing computer, an input device, and an electronic display device, are provided for alternatingly presenting on the display device a stored image of a known good printed circuit assembly and an image of the test printed circuit assembly, whereby defects in the printed circuit assembly under test can be visually identified.
The image processing device, the lighting system, the position control system and the image processing computer can be mounted on a support cabinet that is placed at an appropriate production line location to provide an image capture station. The image capture station includes an image capture station conveyor that forms the upstream part of the image capture and inspection conveyor described above. The image capture station conveyor receives printed circuit assemblies from an upstream portion of the production line and transports them to the image capture location. The image display device and the input device can be mounted on or adjacent to an operator table that is placed downstream of the image capture station to provide an inspection station. The inspection station includes an inspection station conveyor that forms the downstream part of the image capture and inspection conveyor described above. The inspection station conveyor receives printed circuit assemblies from the image capture station conveyor and transports them to a downstream portion of the production line for further processing.
The image processing device of the inspection system is preferably a high resolution digital camera that is mounted at an upper portion of the image capture station. The resolution of the camera is sufficient to acquire a single image of an entire printed circuit board having a size of up to at least about 16xc3x9716 inches at a resolution that is sufficient to allow detailed inspection of each printed circuit component and its connections to the circuit board. In this way, the camera can remain fixed during imaging and complicated camera zooming and positioning equipment is not required. If, during the inspection mode, individual subportions of a test printed circuit assembly need to be inspected, such subportions can be viewed using image processing techniques performed by the imaging control system. More specifically, as described in more detail below, the imaging control system allows users to perform vision comparison on subportions of a test printed circuit assembly in a standard sequence wherein each subportion of the test printed circuit assembly and a corresponding subportion of a known good printed circuit assembly are alternatingly displayed, i.e., animated, for vision comparison inspection. During the standard sequence, animation between the test printed circuit assembly and known good printed circuit assembly subportions continues until the user completes vision comparison inspection on that subportion and signals for the next subportion to be displayed and animated. Users can also stop animation to view the current subportion or any other area of the test printed circuit assembly on a manual basis.
The lighting system is preferably implemented as an array of high lumen, high frequency fluorescent light fixtures arranged on each side of the image capture station, below the digital camera and above the image capture station conveyor, for illuminating a printed circuit assembly positioned at the image capture location. Each light fixture includes a lower light and a upper light such that, in combination, the light fixtures provide a lower bank of low angle lights and an upper bank of fill lights. Each bank of lights is thus disposed around the periphery of an enclosed lighted area that includes the area in which a printed circuit assembly is located when it is within the field of view of the camera.
The position control system may include a retractable stop pin providing a first reference surface for engaging the leading edge of a printed circuit assembly as it is carried on the image capture station conveyor. The position control system further includes at least one clamp associated with the image capture station conveyor for clamping at least one side of the printed circuit assembly against a second reference surface. Importantly, this fixes printed circuit assemblies of all size against a common reference corner and in the same orthogonal orientation. A programmed controller is preferably provided for controlling the movement of the image capture station conveyor, the stop pin and the at least one clamp. The controller advises the imaging control system when a printed circuit assembly is positioned for imaging and is responsive to the imaging control system advising that the printed circuit assembly has been successfully imaged and can be released by the image capture station to the inspection station. The position control system controls the image capture station conveyor to stop when a printed circuit assembly is positioned for imaging and to start when imaging of the printed circuit assembly has been successfully completed.
The imaging control system preferably includes an administrative control module and a user control module. Each module is a software-based interactive processing program that interacts with production personnel through a graphical user interface presented on the output display device. The administrative control module allows an administrator to create and store images of known good circuit assemblies. To facilitate the preparation of such stored images, the administrative control module includes a mask definition function that allows an administrator to define a mask in the known good printed circuit assembly image that bounds the known good printed circuit assembly and hides extraneous image data that is outside the mask-bounded area. The mask is stored in association with the known good printed circuit assembly image in the image processing computer. It is used for clipping subsequent test printed circuit assembly images and the known good printed circuit assembly image so that only image data bounded by the mask is displayed and unnecessary image data outside the mask-bounded area is not displayed. This allows the test printed circuit assembly to be more effectively compared with the corresponding known good printed circuit assembly. If multiple palletized or panelized printed circuit assemblies are captured within a single known good circuit board image and a corresponding test printed circuit assembly image, multiple masks may be defined in the known good printed circuit assembly image to allow vision comparison of each test printed circuit assembly in the test printed circuit assembly image that is bounded by one of the masks.
The administrative control module preferably also provides a fiducial definition function responsive to input from an administrator for defining a pair of fiducials within each mask defined in the known good printed circuit assembly image and for storing the fiducials in association with the known good printed circuit assembly image in the image processing computer. The fiducials can be utilized by users to align each mask-bounded portion of the test printed circuit assembly and known good printed circuit assembly images. To provide a visual guide to administrators, each defined fiducial is designated by a fiducial mark displayed in the known good printed circuit assembly image.
The user control module is preferably adapted to automatically utilize the mask(s) defined in connection with the known good printed circuit assembly image to clip the mask-bounded portion(s) of the test printed circuit assembly image and the known good printed circuit assembly image. The user control module also preferably provides a user-assisted alignment function wherein users are allowed to place fiducials in the mask-bounded portion(s) of the test printed circuit assembly image. The fiducials placed in the test printed circuit assembly image are matched by the user control module with the fiducials previously defined by an administrator for the corresponding known good printed circuit assembly. This brings the mask-bounded portion(s) of the test printed circuit assembly image into alignment with the corresponded mask-bounded portion(s) of the known good printed circuit assembly image. The user control module may also provide a defect log function responsive to input from the user for generating a log of defects found in the test printed circuit assembly. If any such defects are identified, the system requires the user to specify how the defect has been resolved or the test printed circuit assembly will not be released from the inspection station. Optionally, the user may be prompted to identify the source (i.e., production line location) of the defect. After the user has resolved all defects, and has released the test printed circuit assembly from the inspection station, the defect log, along with other useful information, is stored in a defect log file containing records corresponding to the just-inspected printed circuit assembly under test, along with records for other printed circuit assemblies that have been tested. There can be one or more of such defect log files, with each log file preferably storing records of circuit assemblies tested over some time period, such as one month. The defect log files can specify a variety of information for each printed circuit assembly under test, including but not limited to, (1) the printed circuit assembly name or number, (2) a directory path and filename for the printed circuit assembly is stored, (3) a user identifier, a (4) defect type identifier, a (5) defect source identifier, a (6) defect resolution number, a (7) defect location (e.g., pixel coordinates) relative to a known reference point on the printed circuit assembly, and (4) date and time stamps. Optionally, defect logging can be disabled if desired.
The graphical user interface provided by the administrative and user control modules provides a menu-driven display that allows administrators and users to rapidly view and manipulate printed circuit assembly images on the output display device. In both the administrative and user modes a large primary window is provided for displaying an entire printed circuit assembly, or a sub-portion thereof, in one view. An overview window is provided adjacent to the primary window which displays a thumbnail view of the entire printed circuit assembly overlaid with a rectangular view frame illustrating the size and position of the currently-viewed printed circuit assembly sub-portion. Viewing a printed circuit assembly sub-portion in the primary window allows for detailed viewing and is the most common mode in which printed circuit assemblies are inspected for defects.
A user can quickly cycle through each image sub-portion using the standard viewing sequence referred to above. After the standard sequence has completed, users can select subportions manually, for example, by moving the view frame around in the overview window. If necessary, the user can also zoom in or out, pan around and scroll within the primary window in order to inspect any desired area of the test printed circuit assembly. If defects are identified during the user mode, a defect list box is preferably created adjacent to the primary window. As defects are tagged in the primary window, one or more pop-up menus appear that request the user to specify the nature of the defect, its source, and its resolution, as referred to above. Both the defect and resolution information are displayed in the defect list box. A defect tag icon is also placed in the image displayed in the primary window over the component that the user identified as being defective. The color of this icon signifies defect resolution status.